1. Field of the Invention
The present invention relates to a method of forming a semiconductor device having a metal gate, and more particularly to a method of forming a semiconductor device which can prevent against work function layer damage.
2. Description of the Prior Art
Poly-silicon is conventionally used as a gate electrode in semiconductor devices, such as metal-oxide-semiconductors (MOS). However, with a trend toward scaling down the size of the semiconductor device, work function metals are used to replace the conventional polysilicon gate to be the control electrode that cooperates with the high-K gate dielectric layer. The conventional dual metal gate methods are categorized into gate first process and gate last process. Among the two main processes, the gate last process is able to avoid processes of high thermal budget and to provide wider material choices for the high-K gate dielectric layer and the metal gate, and thus gradually replaces the gate first process.
In a conventional gate-last process, after a dummy gate is formed, the dummy gate is removed to forma trench where a work function layer and a metal gate layer are sequentially filled therein to form a metal gate structure. Due to the shrinkage of the device size as a consequence of device miniaturization, it is increasingly difficult to fill the materials into the trench. In some cases, overhang structure and void may occur, thereby decreasing the quality of the metal gate.